The 3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as “LTE”) and LTE-Advanced which is an evolved version of LTE (hereinafter, referred to as “LTE-A”) employ orthogonal frequency division multiple access (OFDM) scheme for the downlink communication scheme.
For frequency scheduling and link adaptation in OFDMA, each terminal (which may also be called “UE (User Equipment)”) measures channel information (CSI: channel state information) and reports the channel information (CSI measurement result) to a base station (which may also be called “eNB”). On the other hand, the base station performs appropriate resource allocation for the terminal using channel information (CSI measurement result).
In LTE, CSI is measured using a cell-specific reference signal (CRS). CRSs are transmitted in all subframes. The terminal can observe CRSs at points in time at which synchronization is established.
On the other hand, in LTE-A, CSI is measured using CSI-RSs (reference signals for channel quality measurement). Since LTE-A is a system which is an extended version of LTE while maintaining backward compatibility with LTE, CSI-RSs, which are LTE-A-specific reference signals, are arranged at a low density in both time and frequency domains so as to minimize insertion losses of CSI-RSs. For this reason, the terminal needs to acquire CSI-RS-related parameters as information broadcast in a cell (broadcast information) by the time CSI-RSs arranged at a low density are observed. As CSI-RS-related parameters, the number of transmitting antennas, positions of time/frequency resources in a subframe, transmission period, and subframe offset or the like are defined.
In LTE, there are two types of operation of CSI measurement and CSI reporting: operation of periodically reporting CSI (hereinafter referred to as “periodic CSI reporting”) and operation of aperiodically reporting CSI (referred to as “aperiodic CSI reporting”).
In periodic CSI reporting, the terminal performs CSI measurement according to one measurement operation indicated beforehand of a plurality of measurement operations in preparation for reporting using resources of a specified uplink (e.g., uplink control channel), maps the CSI measurement result to an uplink control channel and reports it to the base station. One example of the measurement operation is CSI reporting assuming closed-loop MIMO (multiple input multiple output) control. This CSI reporting adopts operation of measuring and reporting, for example, RI (rank indicator) indicating a spatial multiplex number, wideband desired precoding matrix (PMI: precoding matrix indicator), and wideband channel quality information (CQI: channel quality indicator).
In aperiodic CSI reporting as well as periodic CSI reporting, the terminal performs CSI measurement according to one measurement operation indicated beforehand. However, aperiodic CSI reporting is different from periodic CSI reporting in that a terminal reports CSI at a timing instructed from a base station and reports CSI using resources on a common data channel.
An example of the aforementioned method of indicating measurement operation beforehand is a method using a radio resource control message (RRC signaling). Aperiodic CSI reporting is instructed through assignment of an uplink data channel (e.g., PUSCH) using a downlink control channel (e.g., PDCCH).
Different measurement operations may also be indicated between periodic CSI reporting and aperiodic CSI reporting. For example, in periodic CSI reporting, RI, wideband PMI and wideband CQI are reported as described above, whereas in aperiodic CSI reporting, RI, wideband PMI and narrow band CQI can be reported. That is, in aperiodic CSI reporting, narrow band CSI is reported instead of wideband CQI in periodic CSI reporting. In this case, periodic CSI reporting is used for rough link adaptation in which periodic CSI reporting is referenced at the time of non-urgent data transmission, and aperiodic CSI reporting is used for detailed link adaptation in which aperiodic CSI reporting is referenced at the time of urgent data transmission, thus allowing an operation in which the two types of CSI reporting are used for different applications.
In LTE-A, an operation to report two types of measurement targets indicated beforehand through periodic CSI reporting has been added. In LTE-A, another operation has been added whereby two types of measurement targets indicated beforehand in aperiodic CSI reporting are also associated with timings instructed from the base station and reporting corresponding to one of the two types of measurement targets is performed at each timing. The two types of measurement targets are indicated using a bitmap corresponding to 40 consecutive subframes using RRC signaling.
In LTE-A, a concept of carrier aggregation is introduced, which extends the number of bits for indicating aperiodic CSI reporting to two, and can adopt operation of measuring and reporting two types of component carrier groups indicated beforehand in addition to the operation of measuring and reporting component carriers instructed to be reported.
In LTE-A, the introduction of coordinated multiple transmission point (CoMP) is under study. CoMP is a technique whereby a plurality of base stations (cells or transmission points (TP)) cooperate with each other to transmit signals to a terminal (UE) and several schemes are under study. For example, there are two main CoMP schemes under study in 3GPP: (1) CB (coordinated beamforming) scheme and (2) JT (Joint Transmission) scheme.
The CB scheme is a scheme in which only a specific TP stores data intended for a certain terminal. That is, a signal from a TP that stores no data intended for the terminal (e.g., TP adjacent to a TP to which the terminal is connected) is regarded as interference to the terminal. The CB scheme adopts a method of reducing inter-TP interference through control of transmission parameters. More specifically, examples of transmission parameters include precoding, transmission power, modulation scheme and coding rate. By appropriately controlling these transmission parameters, it is possible to weaken signals from an interference TP (TP that possesses no data intended for the terminal) for the terminal while strengthening signals from a desired TP (TP that possesses data intended for the terminal). Strengthening signals from a desired TP and weakening signals from an interference TP can contradict each other depending on the situation, but various proposals are being made taking into account the trade-off between the two.
On the other hand, the JT scheme is a scheme in which data to a certain terminal is shared by a plurality of TPs. Thus, a plurality of TPs can simultaneously transmit signals intended for the corresponding terminal. For this reason, since the terminal can handle signals from other TPs not as interference signals but as desired signals, an SINR observed at the terminal can be expected to improve. Furthermore, improving a method of generating precoding weights at a plurality of TPs as an operation within a network allows a large performance improvement to be achieved.
For such CoMP control, there is a method of observing channel information between each TP and the terminal to be targets of CoMP control and reporting the channel information to the network as channel information in units of TPs.
There is also a heterogeneous network (HetNet) using a plurality of base stations with different scales of coverage areas. The heterogeneous network is a network combining a macro base station that provides a large coverage area (which may also be referred to as “macro cell,” HPN (high power node)” or “macro eNB”) and a pico base station that provides a small coverage area (may also be referred to as “pico cell,” “LPN (low power node)” or “pico eNB”). Regarding the heterogeneous network, studies are being carried out on a method of easily realizing mobility control (handover) using signals in the physical layer by assigning to a pico cell arranged within the coverage area of the macro cell, the same identification number (cell ID) as that of a macro cell. For operation of such a heterogeneous network, a method is under study which uses reference signals for channel quality measurement (CSI-RSs) newly added for an LTE-A compliant terminal (hereinafter referred to as “LTE-A terminal”) to report channel information (CSI) measured by the LTE-A terminal to a network and selects an optimum transmission/reception point according to the propagation situation (e.g., see FIG. 1 and NPL 1).
Moreover, application of CoMP to a heterogeneous network is also under study. For example, by applying CoMP such as CB scheme or JT scheme between LPN1 and macro eNB shown in FIG. 1, receiving quality in the terminal can be expected to improve.
CSI-RSs are used for CSI measurement or reporting during CoMP control. FIGS. 2A to 2C illustrate configuration examples of CSI-RS with respective numbers of transmitting antenna ports. As shown in FIGS. 2A to 2C, CSI-RSs are each defined by a configuration corresponding to the number of transmitting antenna ports (8 ports, 4 ports or 2 ports) of the base station. In FIGS. 2A to 2C, one RB (resource block) is configured of 12 subcarriers and each block shown in FIGS. 2A to 2C represents resources of two OFDM symbols in each subcarrier that are continuous in a time domain (2 REs (Resource Elements)). In each block (2 REs) shown in FIGS. 2A to 2C, CSI-RSs corresponding to two ports are code-multiplexed.
Each terminal acquires information relating to CSI-RS from the base station beforehand. More specifically, the information relating to CSI-RS is, for example, the number of antenna ports (antennaPortsCount), subcarriers within a subframe, and CSI-RS configuration number that identifies an OFDM symbol position (resourceConfig, hereinafter may be represented by “CSI-RS config(i)” or “#i,” CSI-RS config(0) to (19) in FIGS. 2A to 2C), transmission subframe configured of a transmission period and an offset (subframeConfig), and a power ratio (p-C) between reference signals and data signals (see NPLs 2 and 3).
In FIGS. 2A to 2C, CSI-RS configuration numbers are assigned in order in the time direction and in order in the frequency direction at the same point of time. Moreover, as shown in FIGS. 2A to 2C, the same number is assigned to resource starting positions of the respective CSI-RS configuration numbers (starting positions in order of number assignment) between CSI-RS configurations corresponding to the respective numbers of antenna ports. As shown in FIGS. 2A to 2C, a CSI-RS configuration when the number of antenna ports is small constitutes a subset of a CSI-RS configuration when the number of antenna ports is large (may also be called “(nested structure”). It is thereby possible to identify all the respective resources for the respective numbers of antenna ports with a minimum necessary number of numbers while using overlapping numbers in the CSI-RS configuration corresponding to the respective numbers of antenna ports. For example, CSI-RS config(0) with two ports shown in FIG. 2C can be identified as only resources corresponding to two ports (2 REs) from the starting position of CSI-RS config(0) with eight ports (8 REs) shown in FIG. 2A.
Note that a procedure for a base station to indicate, to a terminal, information relating to CSI-RS of each TP beforehand is adopted to observe channel information between each TP to be controlled for CoMP control (hereinafter represented by “coordinating TP” or may also be called “CoMP measurement set”).
There is also a muting technique that makes data of the TP to which the terminal is connected a non-transmission signal in order for the terminal to observe reference signals (CSI-RSs) transmitted from peripheral TPs. More specifically, each of CSI-RS configs (0) to (9) (see FIG. 2B) which are 4-port CSI-RS configuration numbers of the aforementioned CSI-RS configuration numbers is expressed in a bitmap, and the base station indicates to the terminal as to which resource is designated as a non-transmission signal resource. The information on the bitmap type indicating which resource is designated as a non-transmission signal resource is called “non-transmission CSI-RS configuration number list” (zeroTxPowerResourceConfigList) (see NPL 3).
For example, when resources of CSI-RS configs (1) and (2) of CSI-RS configs (0) to (9) are designated as non-transmission signal resources, the non-transmission CSI-RS configuration number list becomes {0, 1, 1, 0, 0, 0, 0, 0, 0, 0}. Here, “1” represents a non-transmission signal resource and “0” represents a resource other than a non-transmission signal resource in correspondence with CSI-RS configs (0) to (9) respectively in order from the leading bit of the non-transmission CSI-RS configuration number list.
The base station indicates, to the terminal, a transmission subframe as well (zeroTxPowerSubframeConfig) configured of a transmission period and an offset like the aforementioned CSI-RS as a subframe in which a non-transmission signal resource is configured. This allows the terminal to identify which resource in which subframe becomes a non-transmission signal resource.
FIG. 3 illustrates positions of non-transmission signal resources (CSI-RS configs (1) and (2)) within a subframe corresponding to zeroTxPowerSubframeConfig configured in a TP to which a certain terminal is connected. In this case, by causing the CSI-RS configuration of a TP positioned in the periphery of the TP to associate with any one of non-transmission signal resources (CSI-RS config(1) or (2) in FIG. 3), the terminal receives no more interference from data from the TP to which the terminal is connected and can secure CSI measuring accuracy when observing CSI-RSs of the peripheral TP as desired signals.
Note that since both CSI-RS transmission and muting are necessary for channel quality measurement of the entire system band, both techniques are applied to the entire system band. More specifically, CSI-RS and muting are applied to all RBs in the system band. On the other hand, since CSI-RS transmission and muting are techniques added for LTE-A, a terminal compliant with only LTE (hereinafter referred to as “LTE terminal”) cannot detect CSI-RS and muting. In order to avoid influences of demodulation performance deterioration of a received signal generated because an LTE terminal cannot detect CSI-RS and muting, an operation (hereinafter described as “Drop”) is defined which does not apply CSI-RS transmission and muting in specific subframes. Specific subframes in which Drop is defined are roughly divided into the following three subframes.
(1) Uplink/downlink pilot signal transmission slot in TDD (time division duplexing) system (TDD (FS type2) special subframe)
(2) Subframe in which CSI-RS transmission causes collision with synchronization signal, PBCH (physical broadcast channel), SIB1 (system information block type 1)
(3) Subframe configured for paging by base station (hereinafter referred to as “paging subframe”)
FIG. 4 illustrates an example of subframes in which synchronization signal (PSS (primary synchronization signal) and SSS (secondary synchronization signal)), PBCH and SIB1 are arranged according to FDD (frequency division duplexing) and TDD.
FIG. 5 illustrates a configuration example of paging subframes according to FDD and FIG. 6 illustrates a setting example of paging subframes according to TDD. In FIG. 5 and FIG. 6, for example, when set value “nB” is oneT, one paging subframe is set for every 10 ms (10 subframes) (10-ms period). Similarly, when “nB” is halfT, one paging subframe is set for every 20 ms (20 subframes) (20-ms period). The same applies to other “nBs.” Thus, a period (frequency) with which a paging subframe is set, that is, a paging transmission frequency is determined in accordance with set value “nB.”